1. Field of the Invention
The present invention relates to an active-matrix type liquid crystal display device having a light shading film.
2. Description of Related Art
Matrix type liquid crystal display devices include active-matrix-addressed twisted nematic liquid crystal display devices (hereinafter referred to as TN-LCDs), as shown in FIG. 22. The TN-LCD has a liquid crystal cell 3 which is disposed between two polarizers 1 and 2. The liquid crystal cell 3 comprises a lower substrate 4 and an upper substrate 5, which are made of glass or the like and which are opposed to each other, and a liquid crystal layer 6. The liquid crystal layer 6 comprises a continuously 90.degree. twisted nematic liquid crystal. A plurality of picture element electrodes 7 are arranged in matrix form on the upper surface of the lower substrate 4, and a lower alignment layer 8 is provided thereon. A plurality of scanning lines, i.e., gate lines (not shown), and a plurality of signal lines, i.e., drain lines 9, are arranged in the upper side on the lower substrate 4. A thin film transistor (TFT) which is also not shown, is provided in the vicinity of each of intersections of the scanning lines and the signal lines. Each of the TFTs is a switching element which is connected to a picture element electrode 7 and to a scanning line and a signal line 9. A common electrode (a counter electrode) 10 is provided on the lower surface of the upper substrate 5 and an upper alignment layer 11 is provided on the lower surface thereof.
When a voltage signal corresponding to picture data is input to a signal line 9 on a column while all TFTs connected to a scanning line on a row are in an ON state as set by a scanning signal being input to the scanning line, a voltage is applied to a picture element electrode 7 through a TFT which is in the ON state, from the signal line 9. As a result, a voltage is applied to the liquid crystal layer 6 between the voltage-applied picture element electrode 7 and the common electrode 10, so that orientation of the liquid crystal molecules to which the voltage is applied, is changed. An optical change caused by the change of the orientation is visualized by the polarizers 1 and 2. Consequently, a desired display, e.g., a black and white display, is obtained.
Deterioration of display quality as a result of the occurrence of disclination is a serious problem in such a TN-LCD and, in particular, in a high-information-content display having numerous picture element electrodes 7. That is, in a TN-LCD having a normally white mode, when a voltage of about 6 V is applied to the picture element electrodes 7, for example, in a picture element portion 12 shown in FIG. 22, the left side of a dotted line 12a comes to a normal display containing a region 12b of the normal tilt domain which has the same tilt direction of liquid crystal molecules as the pretilt direction thereof. The right side of the dotted line 12a comes to an abnormal display containing a region 12c of the reverse tilt domain which causes an optical leakage to form a void. The dotted line 12a illustrated therebetween shows a disclination line which is a boundary between the region 12b of the normal tilt domain and the region 12c of the reverse tilt domain. A plan view of this one picture element portion 12 is shown in FIG. 23. In this figure, the region illustrated by oblique lines is the abnormal display containing the region 12c of the reverse tilt domain which causes the optical leakage to form a void. When such a void is formed in a portion of the picture element portion 12, the contrast of the entirety of the TN-LCD is sharply lowered, so that the display quality thereof is extremely deteriorated.
The occurrence position of such a disclination will be explained as follows. Disclination occurs at positions at which lines in pretilt directions depending upon alignment directions such as rubbing directions, of the lower and upper alignment layers 8 and 11, i.e., tilt angles of long axes of the liquid crystal molecules on both interfaces between the liquid crystal layer 6 and the lower and upper alignment layers 8 and 11, and lines in directions of lateral electric fields generated between the picture element electrode 7 and the scanning line and between the picture element electrode 7 and the signal line 9, cross at right angles. The reason for this is that because the director of a liquid crystal molecule having positive electric anisotropy, i.e., a unit vector in the direction in which the long axis of the liquid crystal molecule is oriented with priority, is oriented along the direction of a localized electric field, directors in right and left sides of the boundary which is formed by the positions at which lines in the pretilt directions and lines in directions of lateral electric fields cross at right angles, are oriented with reverse tilt angles with respect to each other.
Such a disclination is apt to occur in very small-sized picture elements with a small pitch, in an alignment layer which gives a small pretilt angle to the liquid crystal molecules on the interface between the liquid crystal layer and the alignment layer, during a drive at a high temperature because of a pretilt angle smaller than that of a drive at room temperature, and during occurrence of a strong lateral electric field. In particular, the smaller the pitch of the picture elements is, the smaller relative area ratio of the normal display region 12b to the picture element portion 12 becomes. Consequently, the contrast of the display is extremely lowered. When the pretilt angle is small, the reverse tilt phenomenon is apt to occur, and the positions at which lines in pretilt directions and lines in directions of lateral electric fields cross at right angles, i.e., the occurrence positions of disclination, move to inner side in the picture element portion 12. Therefore, the disclination is apt to occur in a TN-LCD which has very small-sized picture elements and in which a high temperature drive is required, e.g., in a device for use in a vehicle such as a car or the like, or in a device for use as a projector, or the like. Conventionally, because each edge of the openings of the light shading films is set on a line which is an equal distance of the maximum transmission distance of disclination, e.g., about twice the gap between the alignment layers 8 and 11 (the cell gap), apart from the adjacent scanning line or the adjacent signal line 9, in order to reduce such a disclination, there has been a problem in that the aperture ratio thereof is extremely lowered.